1. Field of INVENTION
This invention relates to offshore oil and gas drilling and production and, in particular, to apparatus for tensioning marine risers to a floating platform. This invention will be described in connection with production marine risers and tensioned leg floating platforms (TLP), but it is to be understood that this invention may be used with any floating platform and any marine riser, such as a drilling riser.
2. Prior Art
In the production of oil to a tensioned leg floating platform, a conductor or riser has its lower end secured to a connection at the seabed, and its upper end terminated at the platform. Conventionally, this riser is supported by a tensioning force applied to the upper end of the riser.
Because of the relative motion between the platform and the seabed caused by wave and tide actions, some device is required to maintain the tension on the riser while permitting this relative motion.
One such device is described in the U.S. Pat. No. 4,379,657 to Widiner et al. This patent disclosed a modular riser tensioner made up of a plurality of hydraulic cylinders connected to external oil and gas accumulators mounted on a frame to develop the tension force on the riser. This device did not use wire ropes and thus eliminated the problems of wire rope maintenance and breakage.
Another tensioner was disclosed in the U.S. Pat. No. 4,367,981, to Shapiro. In this patent, the riser tensioner was formed within a slip joint and supplied with hydraulic fluid pressure to develop the tension force by external accumulators as did the Widiner et al device.
Still another hydropneumatic riser tensioner is disclosed in the U.S. Pat. No. 4,540,159 to Jordan which did not require connection between the hydraulic cylinders and external gas or oil accumulators to develop the tension force. This device was used as a cable tensioner and had the same problem of wire rope maintenance and breakage as in the prior art. Although this device was self-contained, it also had multiple cylinders; one which contained a supply of compressed gas (a first accumulator), a second or middle chamber which contained hydraulic fluid, (a second accumulator) pressurized by the gas from the gas accumulator and a central chamber (hydraulic cylinder) which housed the piston and piston rod. This device is intended to be permanently mounted in a deck location, built into a rig support structure, with individual control lines.
Accordingly, it is an object of this invention to improve the prior art riser tensioners by providing a single self-contained air and hydraulic fluid accumulator and hydraulic cylinder which is integrated into a modular tensioner unit. An assembly of such tensioning units forms a riser tensioner.
It will also be apparent to those skilled in the art that by having a one piece accumulator/hydraulic cylinder tensioner unit; much of the existing piping can be removed,making the riser tensioner safer and easier to install; that the unit is of reduced size and weight; and that the complete tensioner-unit may be installed or removed with only the connection/disconnection of the control lines.